1. Technical Field
The present invention relates to a switching element for driving display panels using organic electroluminescent (EL) elements, liquid crystals, etc., and a switching element for high-density memories, etc.
2. Background Art
Remarkable progress recently has been made in properties of organic electronic materials. Particularly so-called organic bistable materials, which show switching behavior such that the circuit current is rapidly increased at a certain applied voltage, have been studied with respect to the application to a switching element for driving organic EL display panels, a high-density memory, etc.
FIG. 5 shows an example of current-voltage characteristics of an organic bistable material, which displays the above switching behavior.
As shown in FIG. 5, the organic bistable material has two current-voltage characteristics: (1) a high resistance property 51 (off state) and (2) a low resistance property 52 (on state), and has such a nonlinear response property that under a bias Vb, an applied voltage of Vth2 or more causes transition from the off state to the on state and an applied voltage of Vth1 or less (Vth1<Vth2) causes transition from the on state to the off state to change the resistance value. Thus, by applying a voltage of at least Vth2 or at most Vth1 to the organic bistable material, a so-called switching operation can be carried out. The voltages of Vth1 and Vth2 may be applied as pulse voltages.
Various organic complexes are known as organic bistable materials showing the nonlinear response. For example, R. S. Potember, et al. have experimentally produced a switching element that uses a Cu-TCNQ (copper-tetracyanoquinodimethane) complex to obtain two stable resistance values corresponding to voltage (see R. S. Potember, et al., Appl. Phys. Lett., 34 (1979), 405).
Further, Kumai, et al. have used a single crystal of a K-TCNQ (potassium-tetracyanoquinodimethane) complex to observe a switching behavior due to a nonlinear response (see Kumai, et al., Kotai Butsuri, 35 (2000), 35).
Furthermore, Adachi, et al. have formed a Cu-TCNQ complex thin film by using a vacuum deposition method to clarify its switching characteristics, and have studied its applicability to organic EL matrixes (see Adachi, et al., Oyo Butsuri Gakkai Yokoshu, Spring 2002, Third issue, 1236).
However, switching elements using the above-mentioned organic charge transfer complexes have the following disadvantages.
The switching elements have poor repeatability of their switching behaviors. That is, all the elements do not display the switching characteristics even if they are produced under the same production conditions. Thus, the switching elements are disadvantageous in their lower switching or transition probability.
Though causes of the unevenness have not been clarified, the following reasons are possible. In the transition from the off state to the on state shown in FIG. 5, a charge must be injected from a metal electrode to an organic film. The interface between the organic material film and the metal electrode has microscopic roughness, and it is thought that, in the transition from the off to the on state, the charge is injected into the organic material film due to electrostatic focusing in the rough portion. While the roughness of the interface depends on the flatness of the electrode and the organic material film, it is difficult to control the microscopic roughness, so that the transition voltage is inevitably uneven.